In recent years, there has been often used a member whose cross section perpendicular to its longitudinal direction has a hat shape, (which will be called a hat-shaped cross section member hereinafter), for a structure member of an automobile vehicle body. A hat-shaped cross section member 1 is formed and worked into a shape depicted in FIG. 1, for example, and has a bent portion 2 bent in its longitudinal direction with flange portions positioned outside.
In the case when the hat-shaped cross section member is formed and worked so as to have the bent portion 2 as above, springback ascribable to residual stress occurs, and as indicated by a dotted line in FIG. 2, hanging down in three-dimensional directions occurs in the longitudinal direction based on the bending point. The correction of this hang-down shape cannot be conducted by the correction of springback in a conventional two-dimensional shape (an opening of a U-shaped cross section in a cross section taken along I-I in FIG. 1). Note that an amount of springback is defined to be the value of an amount of hang down in the vertical direction from the desired shape of a tip portion of a product.
As above, in the forming of the hat-shaped cross section member, securing the shape freezing property is a very important technical challenge.